Purple loosestrife (Lythrum salicaria) is an aggressive invasive weed that forms uniform stands that replace diversified native flora, including important waterfowl food plants. Purple loosestrife forms dense stands where it destroys wetlands by reducing water flow and drying out the soil. Many wetlands in North America have major infestations of purple loosestrife. It has no natural enemies on the continent, neither native herbivores nor pathogens, that presently suppress invasive populations of purple loosestrife. Further, its growth is so dense that most wetland wildlife cannot use it as habitat. Moreover, it is not a preferred food for native animals in North America.
A number of characteristics have enabled purple loosestrife to become a problem. A single, mature plant can produce 2.5-million seeds annually. Established plants grow more than 2 meters tall with 30-50 stems forming wide crowns that dominate a plant canopy. Also, a strong rootstock serves as a storage organ providing resources for spring growth and regrowth if the above-ground shoots are destroyed. R. F. Nyvall, Mycologia, 87:501-506 (1995).
The U.S. Department of Natural Resources as well as most states in the U.S. have declared it a noxious weed and require that it be controlled. It is also on the Noxious Weed List in Alberta, Manitoba, and numerous municipalities in Canada. Governmental agencies have been trying to control the spread of purple loosestrife by various means including cutting it, pulling it out, spraying it with herbicides and burning it. Some of the control methods make the situation worse by killing surrounding plants so the purple loosestrife seedlings have no competition. For example, the seemingly obvious solution of pulling it up by the roots does not work. The whole root mass must be removed, causing extensive disturbance to the soil, and creating the very habitat in which this plant thrives. Also, all of the plant must be removed, because it can regenerate from a tiny piece of crown, stalk or leaf buds. Chemical herbicides such as Roundup.RTM. or Rodeo.RTM. are not desirable because they kill essentially all the wetland plants since they are not selective for purple loosestrife.
In addition to there not being acceptable chemical herbicides for purple loosestrife, environmental concerns make biological control a potentially attractive alternative to traditional methods of weed control. Biological control of purple loosestrife began in Germany in the 1960's. It was found that certain species of beetles (Galerucella calmariensis, G. pusilla, and Hylobius transversovittatus) could help control the spread of the weed. In 1991, the Canadian government approved the introduction of these predators of purple loosestrife as biological control agents. In 1992, the United States Department of Agriculture approved the release of these beetles as biological control agents in the U.S.
Unfortunately, it takes quite a long time (3-5 years) for these beetles to become established in an area, if they become established at all. Further, even if they become established in an area, they may not significantly impact purple loosestrife for at least 2-3 years because the plant is so resilient. Moreover, the beetles do not always kill the crown of the plant, so they may not consistently prevent seed production by the plants. The seed bank of purple loosestrife is viable for approximately nine years.
To date, no mycoherbicides have been developed from pathogens of purple loosestrife either in Europe or North America. In fact, disease symptoms are rarely observed in Europe and no pathogens have been isolated from purple loosestrife in Europe with the exception of a nematode Meloidogyne sp. reported to feed on roots in the former USSR. R. F. Nyvall, Mycologia, 87:501-506 (1995).
Thus, there remains a continuing need for a means to safely and effectively control the spread of purple loosestrife. There is further a long-felt, unresolved need to produce an herbicidal composition that can be sprayed, or similarly administered, onto purple loosestrife or other noxious weeds to selectively control the weeds in environments with mixed populations of useful plants, such as in a protected wetland.